As is known, fuel cells enable the direct production of electrical energy by an electrochemical redox reaction between hydrogen (the fuel) and oxygen (the combustion-supporting gas), without proceeding via conversion into mechanical energy. This technology seems promising in particular for automobile applications. A fuel cell generally comprises an association in series of unitary elements each consisting essentially of an anode and a cathode separated by a polymer membrane that enables ions to pass between the anode and the cathode.
As regards the fuel, either a supply of hydrogen is available or the hydrogen needed is produced near the fuel cell by a reformer itself supplied for example with a hydrocarbon. As regards the combustive gas, either the fuel cell is supplied with compressed atmospheric air and excess gas with a reduced oxygen content is discharged downstream from the cell, or the fuel cell is supplied with pure oxygen. This solution has some advantages, in particular a more dynamical response of the cell to a demand for current, which is advantageous in particular for applications in transport means such as automobiles, which are known to impose particularly intermittent operating conditions in contrast to static applications. Other advantages of feeding a fuel cell with pure oxygen which can be mentioned are that the efficiency and the power density are better, and there is no contamination by pollutants present in the atmospheric air.
In this case, however, the fuel cell does not stop operating immediately because one cannot take advantage of the asphyxiating effect of the nitrogen present in air. The electrochemical reaction cannot be interrupted totally by simply turning off the valves through which the fuel and combustive gases are supplied. In effect, the amounts of oxygen and hydrogen that remain trapped in the respective channels of the fuel cell suffice to maintain the electrochemical reaction and there is a risk that the reaction may continue for some hours. Consequently, there will still be an electric voltage across the terminals of the fuel cell.
This phenomenon has several disadvantages:                the persisting electric voltage in itself harbours a risk for people, particularly if an intervention is needed around the fuel cell;        the prolonged and uncontrolled discharge of the gases still present in the cell reduces the pressure in the gas circuits relative to atmospheric pressure and can give rise to pressure differences that may be harmful to the good mechanical condition of the elements of the fuel cell.        
Patent application DE 100 59 393 describes a method for shutting down a fuel cell fed with hydrogen and pure oxygen. That patent application describes the following sequence: first, interruption of the oxygen supply, then use of a variable electric load to dissipate the electrical energy produced by the continuation of the reaction between hydrogen and oxygen in the fuel cell. Thus, when the oxygen pressure has fallen below a predetermined threshold value, the hydrogen and oxygen circuits are flushed with nitrogen to a predetermined pressure. This stops the operation of the fuel cell. However, that solution entails having a reserve of nitrogen. Moreover, the subsequent restarting of the fuel cell is inevitably interfered with by the presence of nitrogen in the gas circuits.